Micromechanically aligned optical assembly

ABSTRACT

An optical assembly includes a combination of laser sources emitting radiation, focused by a combination of lenses into optical waveguides. The optical waveguide and the laser source are permanently attached to a common carrier, while at least one of the lenses is attached to a holder that is an integral part of the carrier, but is free to move initially. Micromechanical techniques are used to adjust the position of the lens and holder, and then fix the holder it into place permanently using integrated heaters with solder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/691,535, filed on Nov. 30, 2012, which is a divisional application ofU.S. patent application Ser. No. 12/698,086, filed on Feb. 1, 2010,which claims the benefit of the filing date of U.S. ProvisionalApplication No. 61/148,551, filed Jan. 30, 2009, entitled“Micromechanically Aligned Optical Assembly” the disclosures of whichare incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates generally to the fiber optic communications, andmore particularly to optical packaging techniques used to align lasersources to optical fibers and other types of waveguides.

Optical fiber communications has generally replaced electrical linksover long distances in the past few decades. In more recent past,optical links are being used at shorter distances to connect servers toswitches and for datacenters. In the future it is expected as data ratesincrease and costs of optics decreases that optics will diffuse intocomputers and the connections within a machine or between processorswill be optical. (see for example Kash et al. SPIE Photonics Westconference 2009 and references cited therein, the disclosures of whichare incorporated by reference herein)

A challenge of fiber optics has been that packaging and alignmentprocesses are considerably more difficult than electrical wiring. Theadvantages are the greater bandwidth and reduced degradation of thesignal with distance. At 10 Gb/s data rates, for the signal to travelmore than 100-300 m in a fiber, single mode fiber is generally needed,with a typical mode size of about 8 microns. Laser sources typicallyhave a mode size of only a few microns. Thus the alignment between thelaser and the fiber through the intermediate optics generally has to bevery high precision, and tolerances on the order of a tenth of a micronare typically required. One great advantage of single mode fiber is thatmultiple wavelengths can be coupled simultaneously to get a parallellink through a single fiber. Thus a 100 Gb/s signal can be sent througha single mode fiber for many kilometers by using ten channels of 10 Gb/seach, with every lane at a different wavelength.

As an alternative, when distances are on the order of 100 m or less,multimode fiber and multimode vertical cavity lasers are often used. Inthis case the core size in the fiber is much larger, at about 50 um, andtolerances can be substantially looser. However, the reach is limited asdifferent modes of the fiber travel at different speeds and it isbecomes more difficult to transmit multiple wavelength simultaneously.

As bandwidth requirements increase, there is increased parallelism inboth single mode and multimode fiber links. In single mode systems,parallel channels can be obtained easily by adding wavelengths to thesame fiber. In multimode systems, additional fibers generally are addedto form a fiber ribbon. Parallel ribbon fibers are of course quiteexpensive and connectors with 24 fibers inside are complicated to make,even if they use multimode fiber with looser alignment tolerance.

There has been considerable work in the industry on different techniquesof loosening the alignment tolerance in single mode systems. However,none is very effective, especially if multiple sources are coupled intothe same fiber. In these cases there are multiple single mode alignmentsthat occur in the same package.

The simplest way to loosen the tolerances slightly is to fabricate alaser with a bigger optical mode. The technique most commonly used is tohave a tapered section at the output of the laser where the optical modeis expanded. This makes the laser mode roughly the same size as theoptical fiber or waveguide mode and the alignment tolerance increasesfrom about a quarter micron to about a micron. The disadvantage of thistechnique is that the fabrication of the laser or semiconductor sourcebecomes more complex, raising the cost. There is also some sacrifice inthe performance of laser. In addition, the effect of a laser with aslightly larger optical mode is not that dramatic. One micron alignmenttolerance is better than a quarter of a micron, however, it is still notamenable to low cost packaging techniques.

Another technique is to etch the facet of the laser and add a passivesilica waveguide. The laser is bonded upside down to a planar lightwavecircuit (PLC) that has waveguides built in. The passive waveguide in thelaser source and the waveguide in the PLC are matched in effectiveindex, and with a slight taper, all the power can theoretically transferfrom the laser source into the single mode waveguide underneath. Thisloosens the tolerance in the die bonding process to about 5 um, allowingthe use of some standard packaging and diebonding equipment. The issuewith this technique is that the laser chips become tremendously morecomplicated. One has to etch a facet and through epitaxial andlithographic processes, align a passive waveguide to the semiconductorwaveguide. Such lasers are highly customized and there is an unavoidableoptical loss between the laser waveguide and the passive waveguideformed next to it.

MEMS with active rather than passive alignment has also been used toalign lasers and waveguides. Alignment may be performed with a MEMSmirror with alignment maintained by a control loop. However, thefeedback loop has to be maintained during operation, requiring that thehigh voltage control electronics outside of the package stay activeduring operation.

There have been some proposals of MEMS active alignment techniques forswitches and alignment of arrays. Some have moving waveguides (E.Ollier, “1\×8 Micromechanical Switches based on Moving Waveguides,” inProc. 2000 IEEE/LEOS Int. Conf. Opt. MEMS Kauai, H I, August 2000, pp.39-40.), some have torsional mirrors (MEMS optical switches, Tze-WeiYeow; Law, K. L. E.; Goldenberg, A. Communications Magazine, IEEE Volume39, Issue 11, November 2001 Page(s):158-163) and some with lenses on anx-y stage (MEMS packaging for micro mirror switches, Long-Sun Huang;Shi-Sheng Lee; Motamedi, E.; Wu, M. C.; Kim, C.-J. Electronic Components& Technology Conference, 1998. 48th IEEE Volume, Issue, 25-28 May 1998Page(s):592-597) (all of which are incorporated by reference herein).However, all of these approaches are complex and difficult to apply, forexample, to PLCs.

BRIEF SUMMARY OF THE INVENTION

In some aspects the invention provides a structure containing multiplelasers of different wavelengths, a planar lightwave circuit that cancombine the different wavelengths into a single waveguide, and set oflenses whose position can at least initially be adjusted usingmicromechanical means all mounted on a submount where

the lasers and the planar lightwave circuit are soldered onto thesubmount

lens holders are an integral part of the submount and are initiallyadjustable

A structure as above where the lens holders are on a lever, therebydemagnifying the motions used to adjust their positions.

A structure as above where the submount contains a region ofpredeposited solder that can be reflowed with an integrated heater, andwhere the solder can lock down the position of the lens with electricalmeans

The structure as above where actuators are formed on the submount, as anintegral part of the submount, and where the actuators move the lensesto optimize the coupling without external mechanical motion

In some aspects the invention provides a structure of at least onelaser, one output waveguide, and a microlens, whereby the microlens canbe moved by electromechanical means and locked down after optimizing thecoupling.

In some aspects the invention provides a structure comprising ofmultiple lasers of different wavelengths, a planar waveguide circuitthat can combine the different wavelengths into a single waveguide, aset of lenses for collimating and focusing the beam, and a set ofmicromirrors, whose deflection can adjust the position of the beam andthe focusing of the beam into the waveguide.

The structure as above where the lasers are mounted ontop of the planarlightwave circuit and emit the optical beam through the microlens ontoto adjustable micromirror.

In one aspect of the invention, the invention provides amicromechanically aligned optical assembly, comprising: a firstwaveguide on a substrate; a second waveguide on the substrate; a lensfor focusing light of the first waveguide into the second waveguide; anda lever holding the lens, the lever having at least one point fixed withrespect to the substrate, the lever holding the lens at a position suchthat movement of the lever will result in demagnified movement of thelens in at least directions other than an optical axis of light of thefirst waveguide, the lever moveable so as to position the lens to focuslight of the first waveguide into the second waveguide.

In one aspect of the invention, the invention provides the assembly ofclaim 1, further comprising: a plurality of further first waveguides onthe substrate; a plurality of second waveguides on the substrate; aplurality of further lenses, each of the plurality of further lenses forfocusing light of a corresponding one of the further first waveguidesinto a corresponding one of the further second waveguides; and aplurality of further levers, each of the further levers holding acorresponding one of the plurality of further lenses, each of thefurther levers having at least one point fixed with respect to thesubstrate, each of the further levers holding the corresponding furtherlens at a position such that movement of each of the further levers willresult in demagnified movement of the corresponding lens in at leastdirections other than an optical axis of light of the corresponding oneof the further first waveguides.

In one aspect of the invention, the invention provides an opticaldevice, comprising: a first optical component configured to providelight; a second optical component configured to receive light; and athird optical component in an optical path between the first opticalcomponent and the second optical component, the third optical componentmounted on an arm with a length along an axis substantially parallel toan axis defined by the optical path between the first optical componentand the third optical component.

In one aspect of the invention, the invention provides a method ofmaking an aligned optical assembly, comprising: manipulating a leverholding a lens to position the lens to focus light from a firstwaveguide into a second waveguide, the first waveguide and the secondwaveguide, being physically coupled to a substrate and the lever havinga fulcrum fixed in position with respect to substrate, the leverdemagnifying movement of the lens in other than an optical axis of thelight.

In one aspect of the invention, the invention provides a method ofmaking an aligned optical assembly, comprising: moving a lever holding alens to position the lens to focus light from a first waveguide into asecond waveguide, the first waveguide and the second waveguide beingphysically coupled to a substrate, with the lever having a point fixedwith respect to the substrate and the lever having a lengthsubstantially parallel to an optical axis of the light from the firstwaveguide to the lens; and fixing position of the lever with the lensfocusing light from the first waveguide into the second waveguide.

In one aspect of the invention, the invention provides amicromechanically aligned optical device, comprising: a first waveguidecoupled to a substrate; a second waveguide coupled to the substrate; alens for focusing light from the first waveguide into the secondwaveguide, the light having an optical axis substantially parallel to aplanar base of the substrate; a holder for holding the lens, the holderphysically coupled to the substrate; at least one electrically actuatedactuator at least partially coupled to the holder, the actuatorconfigured to cause movement of the holder in at least one directionabsent application of means for effectively fixing position of theholder; and means for effectively fixing position of holder.

In one aspect of the invention, the invention provides a method ofaligning an optical assembly, comprising: providing light from a firstwaveguide physically coupled to a substrate; providing an electricsignal to an actuator to move a lens to focus light from the firstwaveguide into a second waveguide, the lens on a holder physicallycoupled to the first substrate, the actuator fixedly physically coupledto the holder; determining that the lens is focusing light from thefirst waveguide into the second waveguide; and fixing position of theholder.

In one aspect of the invention, the invention provides an alignedoptical device, comprising: an input waveguide physically coupled to asubstrate; an output waveguide physically coupled to the substrate; alens configured to focus light from the input waveguide into the outputwaveguide; an arm holding the lens, the arm having a longitudinal lengthsubstantially parallel to an axis defined by a linear path from theinput waveguide to the output waveguide, the arm being fixed in positionwith respect to the substrate.

In one aspect of the invention, the invention provides an alignedoptical device, comprising: an input waveguide physically coupled to asubstrate; an output waveguide physically coupled to the substrate; alens configured to focus light from the input waveguide into the outputwaveguide; an arm holding the lens, the arm having a longitudinal lengthsubstantially parallel to an axis defined by a linear path from theinput waveguide to the output waveguide; and means for fixing positionof the arm with respect to the substrate.

In one aspect of the invention, the invention provides an opticaldevice, comprising: an input waveguide; an output waveguide; a convexmirror mounted in a holder, the mirror moveable, in the absence ofapplication of means to effectively fix position of the mirror, toreflect light from the input waveguide into the output waveguide; an armphysically coupled to the mirror, the arm having a moveable free enddistal from the mirror; and means for effectively permanently fixingposition of the mirror.

In one aspect of the invention, the invention provides an opticaldevice, comprising: a plurality of input waveguides physically coupledto a substrate; a plurality of output waveguides physically coupled tothe substrate; a plurality of lenses configured to focus light from eachof a corresponding one of the input waveguides into a corresponding oneof the output waveguides, the plurality of lenses mounted in a holder; aplurality of arms physically coupled to the holder, the further armsmoveable, in the absence of application of means to effectivelypermanently fix position of the arms with respect to the substrate, soas to cause focus of light from each of the corresponding ones of theinput waveguides into the corresponding ones of the corresponding outputwaveguides; and means for effectively permanently fixing position of thearms with respect to the substrate.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates 4 lasers coupled to a planar lightwave circuit (PLC)with 4 input waveguides using ball lenses on micro-adjustable stages.

FIGS. 2A and 2B show a silicon-on-insulator PLC used for multiplexingwavelengths, each in a different waveguide, into a single waveguide, anda chart of transmittance versus wavelength for an example of the PLC.

FIG. 3 is a schematic of a distributed feedback laser.

FIG. 4A shows schematically demagnification of motion of, for example, alens by a class I lever.

FIG. 4B shows schematically demagnification of motion of, for example, alens by a class II lever.

FIG. 4C shows a handle/lens holder/spring assembly.

FIG. 5 is a plot showing resonance frequency and strain as a function ofvarying spring parameters for a 250 um and a 500 um diameter ball lens.

FIG. 6 shows coupling efficiency of a silicon ball lens compared to acustom aspheric at various lens diameters.

FIG. 7 shows an example variation in coupling efficiency as a functionof lateral misalignment of a 100 um and 300 um ball lens.

FIGS. 8A and 8B show a silicon optical breadboard. In 8A, the originalsilicon-on-insulator cross-section is shown, and in B, the fullyprocessed breadboard.

FIG. 9 shows the breadboard of FIG. 8 populated with lenses, PLC, andlaser diodes.

FIGS. 10A and 10B show a cross-section of a device with predepositedsolder and with the solder melted to lock the micro-lever arm into analigned position. FIG. 10A shows the cross-section prior to alignmentand soldering, while FIG. 10B shows the cross-section after theprocedure is completed.

FIG. 11 shows dependence of coupled power into a waveguide as a functionof the linear displacement of the handle.

FIG. 12 shows the power coupled to a waveguide when this outputwaveguide is shifted to different positions, which illustrates that byactuating the MEMS, the power can be completely recovered.

FIG. 13 shows a lever structure with two springs. This loosenstolerances longitudinally as well as laterally and vertically. If thehandle is compressed or stretched, the lens moves in proportion to theratio of the stiffness of the two springs.

FIG. 14 shows an example of a three axes stage for the alignment of themicrolens. A chevron with integrated heater pushes the lens along theoptical axis, a dual arm thermal actuator moves the lens laterally,while a capactive pad below the arm moves the part up and downelectro-statically.

FIG. 15 shows a lens holder for use with either an array of 4microlenses or 4 individual lenses.

FIGS. 16A and 16B illustrate on embodiment of the invention whereinstead of a PLC with four integrated waveguides, four fibers are used,and a cross-section of a fiber and groove holding the fiber.

FIG. 17 illustrated a single exploded view of a spherical mirrorconnected to an alignment rod, and an array of 4 such devices assembled.

FIGS. 18A and 18B show a side view (FIG. 18A) and a top view (FIG. 18B)of curved mirrors used to refocus the diverging beam of a laser diodeback into the input waveguides of a PLC.

FIG. 19 shows how a combination of a ball lens and a curved mirror canbe used to coupled the light from the laser into the PLC. In this casean isolator can be placed in between the two ends.

FIG. 20 shows an assembly of 4 lasers, four adjustable concave mirrorsand a thin film filter combination to multiplex the beams into a singlefiber.

FIGS. 21A and 21B show the dimensions of an aspheric lens made insilicon that can provide high coupling efficiency for this application.In the inset, the coupling loss is shown as a function of thepositioning error.

FIG. 22 is a flow chart of a process for aligning an optical device inaccordance with aspects of the invention.

DETAILED DESCRIPTION

Aspects of the present invention uses adjustable elements that areintegral to the submount to move the optics to optimize the coupling.Once the alignment is perfected or acceptable, the parts are permanentlysoldered into place using microheaters.

FIG. 1 shows a laser to PLC coupling part in accordance with aspects ofthe invention. The entire assembly is mounted on a silicon breadboard orplatform 10 that has been suitably prepared for hybrid integration. APLC multiplexer 20 that combines many wavelengths into one waveguide ismounted on the silicon breadboard. Such a device could be based onetched gratings fabricated in silicon-on-insulator (SOI), or could bebased on an Arrayed Waveguide Grating (AWG) fabricated with silica onsilicon technology. In either case, there would be a plurality of fouras illustrated, input waveguides 30 at one side of the chip and a singlewaveguide on the other side for output.

There are also 4 lasers 60 soldered on to the silicon breadboard 10.Each laser preferably has a different wavelength, where the wavelengthis matched to that of the input waveguide of the PLC. The diverginglight from each laser, typically with a full width at half maximum of 20degrees in the horizontal and 30 degrees in the vertical is refocused bya ball lens 50 into the input waveguide of the PLC 30. Note that theball lens 50 is preferably placed closed to the laser than to the PLC tomagnify the image and match the farfield to the smaller naturaldivergence of the PLC input waveguides (typically 15 degrees by 15degrees).

Each ball lens 50 fits into a holder etched out of silicon breadboardmaterial. This holder is initially free to move in all three dimensions.There is a handle 90 at the end of this holder that can be manipulatedin all three axes. The other side of the holder is fixed in the siliconbreadboard 10 and cannot move. Between the ball lens and the fixed endof the holder there is a spring or flexture 40 that is made of thinnersilicon in a zig-zag structure, allowing it to stretch slightly and bendup and down. As the handle 90 is manipulated up and down the lens on theholder also moves up and down. The entire spring/lens/holder assembly isa lever, where the lens is placed much closer to the pivot point. Thiscauses a mechanical demagnification, such that a large motion of thehandle causes a smaller motion of the lens.

Since the optical alignment of the system is generally important in thex and y directions (up/down and side-to-side), there is demagnificationin both axes. However, the z or optical axis dimension, the alignmenttolerance is much looser, and thus no demagnification is required. Inthis case the spring 40 stretches or compresses slightly.

There is a small metalized pad on the handle 85 and two thickdepositions of solder on either side of the holder 80. There iselectrical contact by way of metallization 87 (shown in FIG. 10A)between the two deposited solder regions, such that the application ofelectrical current between the solder pads causes localized heating andthe solder to melt and lock the handle in position. Once the lasers, thePLC and lenses have been loaded on to the stage, the lasers areactivated, and the holder 90 is adjusted to maximize the opticalcoupling to the PLC. At an acceptable optical coupling, and preferablyoptimum optical coupling, electrical current is applied to the solderpads, and the solder flows to a position to lock the holder in position.Optical coupling may be evaluated by determining optical output of thePLC, which may be performed for example measuring optical power using anoptical power meter or other device. A substantial advantage of havingthe solder pad at the far end of the assembly is that any mechanicalmotion that might occur as the solder cools down is demagnified, and thesystem will see minimal reduction in output coupling. Generally theelectrical current to melt the solder is removed after the solder hasflowed to position to lock the holder in position, or sufficient heatinghas been applied to allow the solder to so flow. The solder serves, asone of skill understands, as an adhesive. In various embodiments otheradhesives may be used to lock the holder in position, or laser welds orother means may be used.

Once the system is aligned, a high speed driver IC 70 may be mounted ontop of the assembly, although in some embodiments the high speed driverIC is mounted prior to system alignment. This chip would be wirebondedto the lasers and to the silicon breadboard. By keeping the distancebetween the driver IC 70 and the lasers 60 short, good signal integritycan be maintained, and possibly the use of a 50 ohm matching resistorcan be avoided.

There are also electrical interconnects 95 on the silicon breadboardthat take both low speed and high speed signals from the periphery ofthe chip to the driver IC and lasers. The output of the PLC is notshown, but is coupled to a fiber, presumably through another lens andisolator. The entire assembly is then capped with a lid to seal thestructure hermetically. A thick dielectric around the periphery of thechip 98 prevents the cap from shorting the electrical lines and is alsoused under the driver IC to allow room for motion of the lever arms.

FIG. 2A shows such a PLC chip that is commercially available andsuitable for such integration. The chip itself is shown in 200, and thespectral characteristics are shown in FIG. 2B. As previously described,there are 4 input waveguides 30 and a single output waveguide 210.

FIG. 3 shows a distributed feedback laser (DFB) that would beappropriate for this application. DFB lasers are generally available.The laser chip is made on an indium phosphide (InP) substrate 360. Thelight itself is produced in a higher index InGaAsP waveguide core 330.The current is supplied to a top contact 320 and is blocked everywhereby an intermediate layer 350 except to pass through the active stripe.The light is reflected back and forth by a built in diffraction grating310, such that only a single wavelength is fed back and lases. Theselasers have a clean single mode spectrum whose wavelength is preciselydetermined by the diffraction grating pitch. Four such lasers,preferably each with an appropriate wavelength would be used in theinvention. As may be seen in FIG. 3, the DFB is an edge emitting laser.

In aspects of this invention the lens motion is demagnified by the leveron which the lens is placed. FIG. 4A shows schematically a class Ilever. There is a fixed point 410 that acts as the fulcrum for thelever. A lens 50 is mounted on one side of the lever and the motion isimparted to the lever either by an outside actuator or with anintegrated actuator on the other side of the lever, where there may be ahandle 90. There is physical demagnification if the distance between thelens and the fulcrum is less than the distance from the handle to thefulcrum. The lens 50 takes the light from the source waveguide 450 andfocuses it into an output waveguide 30. In one implementation, thesource waveguide is provided by a DFB laser and the output waveguide isprovided by a PLC. An advantage of the lens is that it can effectivelytransform the size of the mode, resulting in a better couplingefficiency. In FIGS. 4A and B, the input waveguide is shown smaller thanthe output waveguide, and thus the lens is closer to the input waveguidethan the output waveguide to magnify the image to match the modes. Whenthe holder side of the lever is actuated upwards, as shown in the figureby the arrow 460, the lens moves downwards, as shown in the figure bythe arrow 470. An advantage of this configuration is that the motion ofthe lens is demagnified by the lever, thus the motion 470 is muchsmaller than the motion 460, and a more precise alignment can beobtained. In a class I lever, the motion of the lens is in the oppositedirection of the motion of the handle

FIG. 4B shows the same concept, but in a class II lever, where thefulcrum or the fixed point 410 is placed at the end of the leveropposite the handle, as is the case of the device of FIG. 1. In thiscase the motion 460 is also demagnified, but in the same direction.

FIG. 4C shows a detail of an embodiment of a spring/holder/handleassembly, useful for the invention. Conceptually, the embodiment of FIG.4C works as a class II lever when a first end 410 of the assembly issubstantially fixed in position, with a first end of the assembly beingthe anchor point or the fulcrum. This end is fixed and connected to thesilicon breadboard. The spring or flexure 40 allows or increases theability of the assembly to bend up and down and side-to-side and tostretch. The lens itself fits into a holder section 420 that has anetched ring 430. Preferably the assembly has a long lever arm 440 thatis substantially larger than the distance from the anchor 410 to thelens holder 420. This allows mechanical motion of a lens in the holdersection to be demagnified. Close to the other side of the lever is ametalized pad 85 that will adhere to the molten solder, allowing thelever to be locked into place. A handle 90 is at a second end oppositethe first end, and may be used to position the lens or mate with anactuator that optimizes the position of the lens.

The design of the spring should preferably be soft enough such thatsufficient motion is obtained in x, y, and z without putting unduestrain on the spring. Similarly the spring should preferably be hardenough such that the assembly does not have a low resonance frequencyand be sensitive to shock and vibration. The spring can be made softerby making the silicon thinner, narrower, or the spring section longer.Similarly, the spring can be made stiffer by varying these dimensions inthe other direction. Nearly all the mass is in the ball lens and theresonance frequency of the assembly can be calculated by knowing thespring constant and the mass of the lens. Similarly the strain on thesilicon can be calculated from the displacement of the lens from theequilibrium. The maximum displacement of the lens is determined by thedie bonding accuracy of the lasers and the PLC. FIG. 5 shows the resultsof this calculation for a 250 um diameter and a 500 um diameter balllens assuming a maximum displacement of the lens of 3 microns,achievable with semi-manual die bonders. On the y-axis, the resonancefrequency of the assembly is plotted and on the x axis is plotted themaximum relative strain in the silicon. The design preferably shouldhave a high resonance frequency to be insensitive to outside shock andvibration. Typically, for resonant frequencies above ˜800 Hz, there areno issues with Telcordia standards. So preferably the design point isabove this number on the y-axis. On the x-axis, if one divides theYoung's modulus of silicon by the yield stress, one obtains that siliconcan theoretically be stretched by 4% before catastrophic failure. Thuson the x-axis, preferably the spring should be below this value ofrelative strain. As FIG. 5 shows, there is considerable freedom in thedesign of the spring, with all the points clear of the design limits. Asthe lens becomes smaller, the design margin increases.

The ball lens should also preferably be designed for optimal coupling.An optimal design matches the laser mode to the PLC waveguide mode. Aball lens is ideal for this application on the order of the low cost andeasy assembly, however, it suffers from increased spherical aberrationcompared to a glass asphere. FIG. 6 shows that the penalty is relativelysmall if one uses small enough optics. Generally aberrations are reducedas the dimensions of the optics decrease. The x-axis in FIG. 6 is thediameter of the lens, while the calculated coupling efficiency is shownon the y-axis. With a custom glass asphere, one can design the surfaceof the lens for optimal coupling, limited only by the mismatch in theelipticity of the modes—in this case above 90%. However, a ball lensfabricated from silicon has high spherical aberrations, and at a 1 mmdiameter has only a 35% coupling efficiency. But as the optics isreduced in size, the coupling efficiencies of the ball lens and theaspheric become similar. The alignment tolerance of small ball lens isslightly tighter than a large optic, but within appropriate limits. FIG.7 shows the alignment tolerance calculation for a 100 micron siliconball lens and a 300 micron lens. As may be seen in FIG. 7 the smalleroptic has better coupling efficiency, but both have similar tolerancesof about 0.15 microns for a 0.5 dB power drop.

FIGS. 8A and 8B relate to the processing steps using lithographictechnologies for a device such as shown in FIG. 1. The starting materialthat ultimately becomes the optical breadboard, shown in A, is a rawsilicon-on-insulator wafer, obtainable from numerous commercial sources.The substrate 830 is n-type silicon, while in this example there is aone micron thick layer of silicon dioxide 820 and a 15 micron thick topp+-type silicon layer 810. The spring and handle will be built from thistop silicon layer.

The wafer is lightly oxidized and then metalized to form the high speedtraces (95). A relatively thick (˜20 um) layer of dielectric is thenformed on the wafer to cover the high speed traces where the cap sealsonto the chip and also form pedestals for the mounting of the driver IC(98). The top silicon wafer is then etched, stopping at the SiO2 layersand forming the cavity around the springs and the handles. The siliconunderneath the oxide is then etched with a KOH solution to undercut andrelease the springs and handles. Note that KOH is selective and will notetch the top p+ doped layer. A final quick oxide etch cleans off anyremaining oxide under the mechanical components. Finally another layerof metallization followed by deposition of solder is applied to form thesolder structure and the metallization on the lever arm. Angledevaporation may be used to allow metallization into the groove under thelever arm.

Once the optical breadboard is completed, the four laser diodes aresoldered into the assembly, with a mechanical tolerance of about <+/−5um. The ball lenses are then fixed to the holders, using for exampleeither solder or high temperature epoxy. Finally the PLC is attachedwith rough alignment of the input waveguides, with a resulting structureas shown in FIG. 9.

A cross-section through one arm around the metallization is shown inFIGS. 10A and 10B before the solder process (FIG. 10A) and after thealignment (FIG. 10B). The assembly is actively aligned, beginning withthe solder in the configuration of FIG. 10A. Each laser is activated andthe handle on the end of the lever arm is adjusted to optimize thecoupling into the PLC. The feedback for the alignment can be obtained bymonitoring the optical power exiting the PLC using an external fibercoupled power meter or an integrating sphere, or alternatively by anon-board photodetector that is mounted on the PLC itself and monitorsthe light in the waveguide. Once acceptable, or preferably optimal,alignment has been achieved, an electrical current is passed between thetwo metalized pads on each side of the moveable arm. This causes thesolder to melt and flow into the cavity and around the arm, sealing orlacking the arm in place, as shown in FIG. 10B.

There are various other ways of fixing the position of the lever afteralignment has been achieved. For example, rather than electricallymelting the solder to lock the arm, one may use a laser to heat thesolder, which may be referred to in the art as laser soldering. One mayalso use epoxies that can be cured either thermally, with UV light, or acombination. Rather than having solder on both sides of the lever, onemay have just one solder ball to one side, and align the part by pushingthe lever into the melted solder ball. Finally, one can fix the arm inposition by laser welding the silicon directly.

After the arm is locked down, the driver IC is attached, the package iswirebonded, the output is coupled to a fiber using a standardmethodology, and a cap put on to seal the package.

The use of the lever discussed earlier is extremely useful in looseningthe alignment tolerances. An apparatus in accordance with aspects of theinvention was built and the alignment tolerances measured with respectto moving the end of the lever. FIG. 11 shows the experimental results.The y-axis of the figure is the optical power coupled to the PLC, whilethe x-axis shows the movement of the lever. The various curvescorrespond the motion in the different axes. The dashed curve 1110 showsthe power coupled into the waveguide if the lens itself is moved in x ory directions (with the z-axis being the optimal axis). As shown in FIG.11, the coupled power drops off rapidly with the linear motion, andtherefore very precise alignment is preferred outside the optical axis.This curve represents the conventional tolerances that one requires infiber optic alignment. However, with the lever, the alignment tolerancesloosen greatly. The curves 1120 and 1130 show the coupled power if thelever is used to perform the alignment in x (horizontal) and y(vertical). As shown in FIG. 11 the tolerances are loosened greatlycompared to the original curve 1110. In this particular structure, thereis no demagnification in z and curve 1140 shows the sensitivity of thecoupled power to longitudinal displacements of the lens. Since this isrelatively loose due to the long depth of focus, there is no need toexpand it, and this curve applies to the conventional alignmenttechnique and the one presented here. In some embodiments, the partssuch as the laser and the PLC are soldered onto the breadboard usingstandard tools with loose positioning tolerance. Once the parts are putdown the levers are moved to align the optical beams, and they arelocked down at the optimal position. The loose tolerance shown in FIG.11 implies that the levers can be moved coarsely, and when they arelocked down, a small shift has negligible effect on the coupled power.However, the fundamental tolerances are even larger than shown in FIG.11, because one can obtain fairly large displacement of the lens usingthis technology. The ability to move the lens implies that parts caninitially be placed with very large errors. FIG. 12 shows measured datawhere the output waveguide was moved horizontally from the optimumposition, analogous to errors in positioning the PLC, and the MEMS wasscanned to re-optimize the alignment. The y axis of the figure isrelative coupled power into the output waveguide. One can see that theoutput waveguide position can be moved +/−12 microns, and the MEMSreoptimized to compensate for the error.

As previously mentioned, the lever described earlier can demagnifytolerances in the horizontal (x) and vertical (y) directions, but doesnot do the same in the longitudinal (z) direction. This is not asignificant problem since the longitudinal tolerance is usuallyrelatively large. However, a simple modification can also allow for alooser tolerance in z. This is shown schematically in FIG. 13. The partis very similar to what was described previously except for the additionof an extra flexing element or spring 1310 on the other side of the lensholder 420. The spring 1310 should preferably be resistant towardsbending, but fairly weak in the longitudinal direction. As the handle 90is pushed in spring 1310 and spring 40 are both compressed. If the twosprings are of equal stiffness, then the lens holder 420 would movetowards the anchor point 410 half the distance that the handle 90 moves,and therefore there would be a demagnification of a factor of 2. Ifspring 310 is 9 times weaker than spring 40, then the demagnificationfactor would be ten times.

In some embodiments actuators for moving the lever are built directly onthe breadboard, including the lever, itself. There are a variety ofactuators that are well known in the art, including comb, thermal, andelectro-static. These can be formed around the lens holder to move thelever in all three axes, and then used to lock the lever into place insome embodiments or hold the lever in place while adhesive, such assolder or epoxy, is used to lock the lever into place. The opticalbreadboard can then be completely assembled and go through an automatedcalibration process, where the on-chip actuators are used to align andthen fix the various adjustable components. This would simplify themanufacture of the part.

An example of a part with built in actuators is shown in FIG. 14. Inthis case the lens holder 420 is connected to a chevron 1400. Thischevron has an insulated metallization formed on the surface of thedevice that comprises a wire bonding pad 1410 on either end and a thinmetal trace on the surface 1420. The lens holder, for example, may becoupled to or connected to the breadboard at both ends of the chevron towhich the pads 1416 are provided. The thin metal trace can be formed outof nichrome or another material that is relatively resistive and canheat up with passing current. Since this metallization is insulated fromthe p+ silicon, it is only thermally connected to the chevron. Ascurrent passes and the chevron heats up, the silicon expands slightly.This thermal expansion is magnified by the geometry of the chevron tomove the lens holder to the left of the figure.

Since the chevron is made of p+ material, it is also electricallyconducting and has two additional pads 1430 on each end. These pads arealloyed into the silicon and passing current between them causes currentto flow in the silicon part itself. The actuator has a central sectionin the chevron 1440 that has an n-type implant and forms a barrier tothe current passing straight through from one pad to the next. Thus theelectric current passes from the top pad through the lens holder 420,through the thick lever arm 440 and then return through the thin leverarm 1450 and down the chevron to the lower bond pad. Since the thinlever arm has a higher electrical resistance, the thin lever arm warmsup and expands slightly compared to the thick lever arm. This causes thetwo arms to bend upwards in the figure in the direction shown in thearrow 1460.

The third actuator that moves the lens down towards the opticalbreadboard (into the page on FIG. 11) is electrostatic. By applying anegative voltage to a pad placed below the actuator 1470, the part iselectrostatically deflected downwards and adjusts the lens in the otherdimension. The embodiment of we have shown a large “handle” 90 at theend of the actuator that would provide a large area to generate a strongdownward force.

In the example of FIG. 14, each actuator moves in one axis, but only inone direction. In such cases, as is well known in the art, thedimensions of the breadboard are such that nominal alignment is obtainedhalf way through the range of motion. That way, by reducing orincreasing the current to the actuators, errors in both directions canbe compensated. In the case where actuators can reverse direction,biasing the part midway through the range would not be necessary.

Once the position of the lens has been optimized by using the threeactuators described above, the part can be soldered or otherwise fixedinto position in the same manner as described previously. Once the partis soldered down, the electrical drive to the actuators is removed andthe part stays in place. There will be some residual stress as theactuators pull back, but the solder should hold the part firmly inplace. Alternatively, one may desire to break off the actuator from thelens holder to eliminate any chance of deformation and creep in thesolder. In FIG. 14, the narrow connection to the lens holder 1480 is aplace that may be utilized for disconnection. Alternatively, the lensholder could be pushed against the actuator with a counter force spring.Electrical actuation would push the lens holder away and compress thecounter force spring. Once the part is locked down, the actuator wouldretract, but the lens holder would stay in place.

When the number of channels becomes larger, integrated arrays can besimpler than using individual components. For example, a single laserchip can contain a number of laser elements, each of which is designedto operate at a different wavelength. Similarly, a microlens array canbe fabricated with precise spacing between the elements. Thus all threeelements, the laser array, the microlens array, and the PLC inputwaveguide array are all matched. In this case the entire microlens arraycan be aligned in one step. FIG. 15 shows a single lens holder intendedfor connection to the breadboard about free ends of springs in someembodiments, with positions for separate lenses. Alternatively, alithographically defined lens array could be mounted on the part. Ratherthan 4 separate handles and four springs, there are only two handles1510, 1520 and two springs 1530, 1540 since the spacing between thelenses do not require adjustment. In this case by pushing both handlesin one direction or the other the entire assembly moves together.However, one can also tilt the structure by pushing the handles indifferent directions.

There are also applications where the PLC is not needed, or is alreadyfiber coupled outside of the package. In these instances the beam iscoupled into multiple fibers directly. In some embodiments four fibersare used that compose a ribbon instead of a PLC. FIG. 16 shows thisembodiment. The PLC has been replaced by four fibers 1610. One advantageof a silicon breadboard is that v-grooves or other alignment structurescan be fabricated in the breadboard to guide the fibers to the correctposition. In this case there is a shallow v-groove etched in the siliconbreadboard. 1620. The enlargement to the side of the figure shows howthe anisotropically etched v-groove holds the fiber. Such v-groovemechanisms are well known in the art. If desired an isolator can beplaced before the fiber, and other configurations, such as a dual lensesin each optical path to collimate and then refocus the beam arepossible.

The discussion so far has centered on the use of a moveable microlens orball lens to optimized the alignment. However, a curved mirror may beused instead. The curved mirror can be moved electrostatically, like astandard micromirror, or be rotated manually with demagnification tosteer the focused beam and optimize the alignment. The curved mirror canbe stamped out at the end of a pin, as shown in FIG. 17. Each mirrorsurface 1710 is curved as to reflect and focus the beam. The mirror ismounted on a rod 1720 that can be manipulated, for example from an enddistal from the mirror. The pin, including the mirror is mounted in aholder 1730, such that it can be rotated to change the angle. An arrayof such mirrors can easily be fabricated 1740, and the angle of each pinindividually adjusted by moving the end of the lever 1750. Note that byhaving a long rod 1720 at the back of each pin, the same demagnificationeffect that was realized in the earlier geometry is once again obtained.Similar methods of fixing the part in place after alignment can be used.

The optical design is shown in FIG. 18. The laser output simply reflectsfrom the curved mirror and is refocused onto the input waveguide of thePLC. The laser(s) can be mounted on top of the PLC.

A combination of ball lenses and curved mirrors can also be used. Thiscan yield to higher coupling efficiency and allow more room for placingcomponents. For example, as shown schematically in FIG. 19, a surfacemounted isolator 1910 could be placed between the ball lens and thespherical mirror. If it has a large enough aperture, it couldaccommodate all four beams. In this case the ball lens 50 collimates thebeam from the laser diode, and the curved minor 1510 refocuses the beamonto the PLC. If the PLC has an angled facet in the proper direction1920, the coupling can be improved.

The configuration of FIG. 19 expands the room available for othercomponents. For example, one could use thin film filters that allow onewavelength to pass through while reflecting other wavelengths could beused in this configurations to multiplex all the wavelengths together.This obviates the need for a PLC, and one may couple directly to theoutput waveguide. This configuration is shown in FIG. 20. Four laserdiodes 60 emit optical beams that are collimated by ball lenses 50.These four collimated beams then pass through three thin film filters2010 that reflect other wavelengths except the one impinging directlyfrom the laser. The beams are then reflected against the sphericalmirrors and are ultimately focued into an optical fiber 2020. Onceagain, is that the entire package can be aligned by moving the leversconnected to the concave mirrors.

For the sake of simplicity the optics have generally been described asspherical, whether it be the lenses or the mirrors. Of course, asdescribed earlier, an asphere can have lower aberration and will resultin higher coupling efficiencies. A very convenient lens for thisapplication is a plano-convex silicon lens that is fabricatedlithographically on a wafer. These lenses are available commercially andgenerally formed by reflowing photoresist or polymer on silicon followedby a dry etch step that transfers the shape of the photoresist to thesilicon. The lens is then antireflection coated on the front and theback and diced either into singlets or arrays. Sometimes these lensesare fabricated on silicon-on-insulator wafers, which are then releasedto form very small lenses. FIG. 21A shows such a lens and FIG. 21B showa chart of a calculation of the coupling using the lens from a laser toa PLC.

In some embodiments to optimize the power into an output waveguide forlight from an input waveguide for devices as discussed herein, somelight is first detected in the output waveguide. When the beam is wellfocused, the spot is quite small and it can be difficult to detect anycoupled light in the output waveguide if x and y positions are notoptimized, with x and y directions being orthogonal to each other andthe z direction, which is generally along an optical axis of light.However, if for example a lens between the input waveguide and theoutput waveguide is far from an optimum position in the longitudinal (z)direction, the beam is very poorly focused and larger, and therefore atleast some light is likely to be detected in the output waveguide. Thus,in some embodiments the lens is first moved almost to the maximumposition along the optical axis, either all the way towards the outputwaveguide, or all the way towards the source waveguide. Then the coupledpower in the output waveguide is measured and recorded as “A”. The lensis then optimized in x, an axis perpendicular to the direction of light.The optimization can be performed by moving the lens in the positive xdirection a small amount, measuring the light, and then moving in thenegative direction a small amount and measuring the light. If at eitherpoint the coupled optical power is larger than at the center point, thisnew point becomes the center point and the process repeats. This cyclecontinues until one is sure that the center point has the maximumcoupled power. The process is then repeated in exactly the same way iny, the other axis perpendicular to the propagation of light, and then inz, the longitudinal or propagation direction. Once all three axes arealigned, the coupled power is measured again and compared to the valueoriginally recorded as A. If the power has increased, the entire processrepeats. On the other hand, if alignment in x, y, and z does not resultin further power increase, then one can be assured of maximum power, andthe process then locks down the lens by epoxying or soldering the leverdown to the substrate.

In some applications, one may not desire to have the maximum powercoupled, since it may lead to modulation powers above the targetspecifications. Furthermore, the power can sometimes not be reduced bydecreasing the laser current, since it can lead to a slower responsefrom the laser. If this is the case, or even if this is not the case,after optimization, the lens can be moved in the z direction away fromthe optimum position to lower the output waveguide coupled powergradually from the maximum amount until the power reaches the desiredvalue.

FIG. 22 is a flow chart of an example of a control loop that can be usedto optimize the power coupled from the source waveguide to the output.The process of FIG. 22 may be performed using a controller or circuitryelectronically coupled to circuitry for determining optical power and toactuators of an optical device, for example as described herein.

In block 221 the process moves the lens to a position along the opticalaxis, which may be considered the z-axis, closest to an outputwaveguide, although in some embodiments the process moves the lens to aposition along the optical axis farthest from the output waveguide, withthe process considering the resulting position in the x,y, and z axisthe x, y, and z center points. In block 2215 the process determines aninitial indication of optical power coupled to the output waveguide.

In block 2220 the process moves the lens to a position a small distancein a first direction perpendicular to the optical axis, with the firstdirection being considered the x-axis, and to a corresponding positionin the opposite direction, and determines an indication of optical powercoupled to the output waveguide for both positions. In some embodimentsa small distance along the x-axis may be on the order of 0.1 microns, insome embodiments about 0.1 microns, and in some embodiments 0.1 microns.In block 2225 the process determines if either measured power is greaterthan the initial indication of optical power coupled to the outputwaveguide. If so, the process in block 2230 sets the position with thegreatest power as the x center point, and the process returns to block2220. Otherwise, the process continues to block 2235.

In block 2235 the process moves the lens to a position a small distancein a second direction perpendicular to the optical axis and the x-axis,with the second direction being considered along the y-axis, and to acorresponding position in the opposite direction, and determines anindication of optical power coupled to the output waveguide for bothpositions. In some embodiments a small distance along the y-axis is asdiscussed with respect to the x-axis. In block 2240 the processdetermines if either measured power is greater than the initialindication of optical power coupled to the output waveguide. If so, theprocess in block 2245 sets the position with the greatest power as the ycenter point, and the process returns to block 2235. Otherwise, theprocess continues to block 2250.

In block 2250 the process moves the lens to a position a small distancein the z-direction, and, if possible, to a corresponding position in theopposite direction, and determines an indication of optical powercoupled to the output waveguide for both positions. In some embodimentsa small distance along the z-axis is five times the small distancediscussed with respect to the x-axis. In block 2255 the processdetermines if either measured power is greater than the initialindication of optical power coupled to the output waveguide. If so, theprocess in block 2260 sets the position with the greatest power as the zcenter point, and the process returns to block 2250. Otherwise, theprocess continues to block 2270.

In block 2265 the process determines an indication of optical powercoupled to the output waveguide, although it should be realized thatmeasured power from block 2250 may be used. In block 2270 the processdetermines if the optical power determined in block 2270 is greater, insome embodiments, and substantially greater, in other embodiments, thanthe optical power determined in block 2215. If so, the process sets theinitial indication of optical power coupled to the output waveguide tothe optical power determined in block 2265, and the process returns toblock 2220. Otherwise the process locks position of the lens, by lockingposition of an arm or lever holding the lens in most embodiments, andthereafter returns.

Aspects of the invention therefore include a platform wheremicromechanically adjustable optical components are used to align fromone or multiple lasers or laser arrays into a planar lightwave circuitor other output waveguides. Although the invention has been describedwith respect to various embodiments, it should be recognized that theinvention includes the novel and non-obvious claims supported by thisdisclosure.

The invention claimed is:
 1. A method of making an aligned opticalassembly, comprising: manipulating a lever holding a lens to positionthe lens to focus light from a first waveguide into a second waveguide,the first waveguide and the second waveguide, being physically coupledto a substrate and the lever having a fulcrum fixed in position withrespect to substrate, the lever having distance between the fulcrum andthe lens being less than distance between the fulcrum and a handle ofthe lever such that movement of the handle of the lever demagnifiesmovement of the lens in other than an optical axis of the light, ascompared to the movement of the handle of the lever.
 2. The method ofclaim 1, further comprising fixing position of the lever.
 3. The methodof claim 1, wherein the lever and the substrate are integrally formed ofthe same material.
 4. A method of making an aligned optical assembly,comprising: moving a lever holding a lens to position the lens to focuslight from a first waveguide into a second waveguide, the firstwaveguide and the second waveguide being physically coupled to asubstrate, with the lever having a point fixed with respect to thesubstrate and the lever having a length and a width, the length of thelever being longer than the width of the lever, with the length of thelever substantially parallel to an optical axis of the light from thefirst waveguide to the lens; and fixing position of the lever with thelens focusing light from the first waveguide into the second waveguide.5. A method of aligning an optical assembly, comprising: providing lightfrom a first waveguide physically coupled to a substrate; providing anelectric signal to an actuator to move a lens to focus light from thefirst waveguide into a second waveguide, the lens on a holder associatedwith an arm, the arm physically coupled to the substrate and having alength and a width, the length being greater than the width andextending substantially along an optical axis of light from the firstwaveguide to the lens, the actuator fixedly physically coupled to thearm; determining that the lens is focusing light from the firstwaveguide into the second waveguide; and fixing position of the arm. 6.The method of claim 5, wherein determining that the lens is focusinglight from the first waveguide into the second waveguide comprisesdetermining that the lens is focusing light from the first waveguideinto the second waveguide so as to optimize coupled optical power to thesecond waveguide.
 7. The method of claim 5, wherein determining that thelens is focusing light from the first waveguide into the secondwaveguide comprises determining that the lens is focusing light from thefirst waveguide into the second waveguide so as to obtain a desiredcoupled optical power to the second waveguide.